Okay, you're right, that does sound like a better idea. I officially change my vote to #3 (that is, to sort by k and, by extension, load files into the server by k).

Oh, and by the way: sorting by k should still be just as much of a stress test for the server. Maybe not quite as prolonged of a stress test, but that shouldn't make a difference--it will still tell us unequivocally whether the server can handle n=50K tests.